This invention relates to electric lamps and, more specifically, to fluorescent electrodeless lamps operated at low and intermediate pressures without the use of ferrite at frequencies from 50 kHz to 200 MHz.
Electrodeless fluorescent lamps utilizing an inductively coupled plasma were found to have high efficacy and lives that are longer than that of conventional fluorescent laps employing hot filaments and internal electrodes. The plasma that generates UV and visible light is produced in a glass (or quartz) envelope filled with inert gas such as argon, krypton or the like and metal vapor such as mercury, sodium or the like. The induction coil that generates the inductively coupled plasma is positioned in the close proximity of the lamp envelope.
In a co-pending U.S. patent application Ser. No. 09/420,673 by Popov et al. (owned by a common assignee) the envelope is a straight or bent single tube with sealed both ends. The induction coil is wound along the tube surface in its axial direction forming several turns (windings) that are parallel to each other and to the tube""s axis thereby generating the axially uniform plasma along the tube walls. The inner walls of the envelope are coated with phosphor and protective coatings. The UV light is absorbed in the phosphor and then converted to visible light uniformly throughout the envelope surface. The efficacy of such a lamp at a frequency of 0.4-15 MHz and RF power of 50-150 W is high, 80-100 LPW, and dependent on lamp power efficiency and phosphor composition.
The plasma is kept axially uniform at lamp dimming down to 20% of the maximum light output that makes the lamp useful at such applications as tunnel lighting and general street lighting. However, in some applications where there are restrictions on bulb size, the use of high RF power, needed for high lumen output, leads to high bulb wall loading (wall surface power density, P/S) exceeding the loading of 200 mW/cm2. The excessive wall loading (higher than 200 mW/cm2) causes phosphor damage and eventually leads to phosphor coating degradation and hence, short lamp life.
For instance, the lamp with the tube of 50 mm in diameter and 300 mm in length, and operated at RF power of 150 W, has the wall loading of 318 mW/cm2. This value is substantially higher (60%) than the generally accepted maximum wall loading of 200 mW/cm2. To reduce wall loading one has to increase the tube surface area or increase the lamp efficacy. However, in many applications, the bulb diameter and length can not be increased due to limitations imposed by the size of the lamp fixture.
As to the lamp efficacy, the substantial increase of the lamp efficacy (by 20-30%) can be achieved by using phosphor producing xe2x80x9cgreenishxe2x80x9d light. However, in most outdoor and indoor applications lamps with xe2x80x9cgreenishxe2x80x9d light are not acceptable.
We found out a new method to decrease substantially the wall loading of the lamp described in the Popov et al patent application without the increase of bulb diameter and length and without sacrificing lamp efficacy. We achieved this by the introduction of a reentrant cavity along the bulb axis that increases substantially the bulb surface area exposed to the discharge plasma and thereby reduces the wall loading, in conjunction with the use of an outer coil and no ferrite.
According to the present invention, a novel approach is disclosed that results in an efficient ferrite-free electrodeless lamp that is operated at a frequency from 50 kHz to 200 MHz with the wall loading below 200 mW/cm2. The lamp power efficiency and efficacy were found to be comparable to those of electrodeless fluorescent lamp disclosed in the Popov et al patent application mentioned above. The present invention comprises an electrodeless fluorescent lamp having a glass envelope made from a straight single tube or bulb of any cross section and size. A reentrant cavity is disposed along the tube axis and sealed to the one of the end of the envelope. A filling of inert gas and vaporous metals such as mercury or cadmium and sodium are placed in the envelope. The metal vapor pressure is maintained below 5 Torr, and inert gas pressure is below 20 Torr. A protective coating is disposed on the inner surface of the envelope and reentrant cavity walls and a phosphor coating is disposed on the protective coating. The reflective coating is disposed on the inner (vacuum) walls of the reentry cavity, between the phosphor and protective coatings thereby reflects visible light from the cavity and reduces light xe2x80x9ctrappedxe2x80x9d inside the cavity. The reflection effect is increased when the outer surface of the cavity walls is also coated with the reflective coating (alumina).
An induction coil formed from the plurality of windings (turns) is disposed on the outer surface of the envelope along its axial direction. All turns are parallel to each other and to the tube""s axis. The radio-frequency (RF) power source coupled to the induction coil generates an RF voltage across the coil that ignites and maintains an RF inductively coupled discharge inside the tube along its walls. The discharge forms a xe2x80x9cclosed-loopxe2x80x9d path inside the tube along its walls that is necessary condition to maintain an inductively coupled discharge in the tube.
The power absorbed by the plasma is partially xe2x80x9ctransformedxe2x80x9d into the power xe2x80x9cdepositedxe2x80x9d by the plasma ions and UV photons in the phosphor coatings on the inner walls of the envelope and the reentrant cavity.
An object of the present invention is to design an efficient ferrite-free electrodeless fluorescent lamp operated in a wide range of frequencies, from 50 kHz to 200 MHz and wide range of power, from 5 W to 2000 W.
Another object of the present invention is to design an envelope that has a reentrant cavity that results in the reduction of the envelope wall loading without the increase of the outer envelope dimensions.
Yet another object of the present invention is to design an induction coil that consumes insignificant amount of RF power in kHz and MHz ranges, so the efficiency of the lamp is the same or comparable to that of a lamp described in the Popov et al application.
Again, another object of the present invention is to locate the coil in such a manner as to provide its efficient coupling with the lamp plasma.
Another object of the present invention is to provide a compact fluorescent lamp operating at kHz and MHz ranges of frequencies, without the use of ferrite and having long life.
A further object of the present invention is to generate axially uniform plasma that generates axially uniform visible light when operated at any power including that of dimming conditions.
The many other objects, features and advantages of the present invention will become apparent to those skilled in the art upon reading the following specifications when taken in conjunction with the drawings and claims.